Perhaps the loudest and most credible of the bold prognosticators is Wharton School economics professor Jeremy Siegel, who’s predicting 15,000 — maybe even 17,000 — by the end of next year.

Siegel told Barron’s in a February cover story that he bases his calculations on 141 years of stock cycles and on the fact that earnings grew at double-digit rates through 2011 and have stayed strong so far this year.

The market, he told the magazine, is due for a dramatic upswing. By late 2013, the odds are two out of three for a Dow 15,000 and 50-50 for 17,000.

A few months farther into 2012, fewer experts are scoffing at projections like Siegel’s.

“While Dow 15,000 and 17,000 may sound like dramatic targets,” Barron’s reports, “from at least two perspectives — earnings and inflation — they are actually rather modest objectives.”

Siegel, meantime, is standing firm on his forecast, defending it again just the other day in an interview with CNBC.

“Even before Apple (which stunned Wall Street with an earnings report that far exceeded expectations) … earnings are up 10 percent year-over-year. Expectations were all down to 2 percent.”

Siegel’s not alone on that one.

Mike Holland, founder of New York-based Holland & Co., says in a Bloomberg Businessweek article that while many market observers are focusing on Apple, they’re missing the broadest market advances since at least 1990.

“Companies have done extremely well, and yet the valuations have gone down because of all of the headlines,” he tells Bloomberg.

The Bloomberg article also notes that 458 of the 479 stocks that have been in the S&P 500 since March 2009 have gone up.